Such informations are never available to , for exactly the reasons you think they should be .
If everyone would, know exactly the sales figures and statistics, then it wouldn’t make market more fair, but would collapse on it’s self. As the prices would most likely get to down the point where most people would think it’s not longer reasonable to make any content.
If you are selling content though, you will get sales statistics (when they are available). What you will made out of them is another matter.
I know it can be frustrating. But like all trade business, you must take leap of faith and/or ask people around. Some will tell you something, some won’t.